1:45 PM - 2:00 PM
[SSS05-01] Strain-Energy Release in the Coseismic and Postseismic Period of the 2016 Kumamoto Earthquake
Keywords:Strain energy drop, Inland earthquakes, 2016 Kumamoto earthquake, Earthquake cycle
The quantitative analysis of coseismic deformation and afterslip following large earthquakes typically involves the evaluation of the seismic moment release. However, since the stress drop associated with afterslip should be significantly smaller than that of the mainshock, the seismic moment fails to accurately represent the amount of energy released (Saito and Noda, 2022). Understanding the energy balance during and after an earthquake is fundamental to elucidate the mechanism driving afterslip. Thus, to deepen our understanding of the mechanics of the coseismic and postseismic slips following inland earthquakes, we estimate the stress change distributions and quantify the minimum released strain energy in the coseismic and postseismic periods of the 2016 Mw7.0 Kumamoto earthquake.
To reliably estimate the stress change and the resultant energy balance during and after the earthquake, we first estimate kinematic models of the coseismic and postseismic slip 1 year after the earthquake in an elastic half-space using the method of Yabuki and Matsu'ura (1992). For this purpose, we utilize GNSS observations from 83 GEONET sites in and around the research area. Transient deformation following large earthquakes includes the effects of afterslip and viscoelastic relaxation. Therefore, to isolate the effect of afterslip, we calculate postseismic deformation due to viscoelastic relaxation using a layered viscoelastic half-space model (Wang et al. 2006) under the assumption of a viscoelastic Maxwell rheology and subtract it from the postseismic time series. Results at this stage are then used to construct the shear stress response on the fault planes in the direction of the slip and the stress drop during and after the earthquake. Finally, we compare the magnitude of the coseismic and postseismic deformation after the earthquake in terms of the minimum strain energy released from the elastic lithosphere or minimum strain energy drop (Kanamori 1977; Saito and Noda, 2022).
Considering a rigidity of 30 GPa, the total seismic moment during the coseismic and postseismic period of the 2016 Mw7.0 Kumamoto earthquake is 4.03x1019N·m (Mw 7.0) and 2.65x1018 N·m (Mw 6.2), respectively. The average stress drops in the coseismic and postseismic periods are 9.5 MPa and 0.5 MPa. The minimum strain energy drop by the mainshock and afterslip is 6.20 x1015 J and 2.13 x1013 J, respectively. While the seismic moment indicates that ~5% of the seismic moment of the mainshock is released as afterslip, only less than 1% of the minimum strain energy was released by the afterslip. On the other hand, failing to account for viscoelastic relaxation in the analysis results in a stress drop of ~9MPa one year after the earthquake, equivalent to that of the mainshock. Careful analysis of the transient deformation following large earthquakes must be performed to reliably estimate the energy balance through the earthquake cycle of inland earthquakes.
To reliably estimate the stress change and the resultant energy balance during and after the earthquake, we first estimate kinematic models of the coseismic and postseismic slip 1 year after the earthquake in an elastic half-space using the method of Yabuki and Matsu'ura (1992). For this purpose, we utilize GNSS observations from 83 GEONET sites in and around the research area. Transient deformation following large earthquakes includes the effects of afterslip and viscoelastic relaxation. Therefore, to isolate the effect of afterslip, we calculate postseismic deformation due to viscoelastic relaxation using a layered viscoelastic half-space model (Wang et al. 2006) under the assumption of a viscoelastic Maxwell rheology and subtract it from the postseismic time series. Results at this stage are then used to construct the shear stress response on the fault planes in the direction of the slip and the stress drop during and after the earthquake. Finally, we compare the magnitude of the coseismic and postseismic deformation after the earthquake in terms of the minimum strain energy released from the elastic lithosphere or minimum strain energy drop (Kanamori 1977; Saito and Noda, 2022).
Considering a rigidity of 30 GPa, the total seismic moment during the coseismic and postseismic period of the 2016 Mw7.0 Kumamoto earthquake is 4.03x1019N·m (Mw 7.0) and 2.65x1018 N·m (Mw 6.2), respectively. The average stress drops in the coseismic and postseismic periods are 9.5 MPa and 0.5 MPa. The minimum strain energy drop by the mainshock and afterslip is 6.20 x1015 J and 2.13 x1013 J, respectively. While the seismic moment indicates that ~5% of the seismic moment of the mainshock is released as afterslip, only less than 1% of the minimum strain energy was released by the afterslip. On the other hand, failing to account for viscoelastic relaxation in the analysis results in a stress drop of ~9MPa one year after the earthquake, equivalent to that of the mainshock. Careful analysis of the transient deformation following large earthquakes must be performed to reliably estimate the energy balance through the earthquake cycle of inland earthquakes.